During radio transmissions, signals routinely reflect off of surfaces. In certain situations, reflected signals travel a path that is longer than the direct, line-of-sight path by some multiple of the radio wavelength that causes destructive interference of the carrier wave at the receiver (multipath nulling). For moving transmitters, such as in aircraft, drones, missiles, and cars, as the distance to the receiver changes, the differential distance between the line-of-sight path and the reflected path dynamically changes such that the destructive interference periodically recurs resulting in periodic nulling of the carrier wave.
Reflection is particularly severe when flying low over calm water and communicating over long distances with a low grazing angle. Under such conditions the two signals will arrive completely out of phase and thereby cancel out each other at the receiver. Such conditions are particularly problematic for data links to sea skimming weapons.
For example: when conducting a low altitude attack on a ship at sea, nulling of communications repeatedly occurs as the plane approaches the target. Even at the speeds of modern jet fighters, the multipath problem causes a complete loss of communications for three to five seconds. For slower airborne vehicles, the loss of connectivity would be proportionally longer.
Frequency hopping has been employed to mitigate multipath nulling, but frequency hopping is not viable where there are a fixed number of available frequencies or the waveform does not lend itself to frequency hopping.
Consequently, it would be advantageous if an apparatus existed that is suitable for preventing periodic multipath carrier wave nulling.